1. Field of the Invention
The present invention relates to a rotating field sensor for detecting an angle that the direction of a rotating magnetic field forms with respect to a reference direction.
2. Description of the Related Art
In recent years, rotating field sensors have been widely used to detect the rotational position of an object in various applications such as detecting the rotational position of an automotive steering wheel. Systems using rotating field sensors are typically provided with means (for example, a magnet) for generating a rotating magnetic field whose direction rotates in response to the rotation of the object. The rotating field sensors use magnetic detection elements to detect the angle that the direction of the rotating magnetic field forms with respect to a reference direction. The rotational position of the object is thus detected.
Among known rotating field sensors is one that includes two bridge circuits (Wheatstone bridge circuits), as described in U.S. Patent Application Publication Nos. 2012/0053865 A1 and 2002/0006017 A1. In such a rotating field sensor, each of the two bridge circuits includes four magnetoresistive (MR) elements serving as magnetic detection elements, and outputs a signal responsive to the direction of the rotating magnetic field. The output signals of the two bridge circuits are different in phase from each other by ¼ the period of the output signals of the bridge circuits. The angle that the direction of the rotating magnetic field forms with respect to a reference direction is determined based on the output signals of the two bridge circuits.
JP S61-173113A discloses a magnetic rotation sensor including two sets of three sensing sections. In this sensor, the three sensing sections in each set are placed in parallel to each other and connected in series such that the spacing between every adjacent sensing sections is ⅓ the write wavelength of a signal magnetic field. A power supply voltage is applied across each set of three sensing sections, and signals are output from two junctions of the three sensing sections in each set. The two sets of three sensing sections are formed on one substrate such that corresponding sensing sections in the two sets are parallel to each other with a spacing therebetween of ½ the write wavelength of the signal magnetic field.
JP H04-005571A discloses a rotating direction discriminating and rotation detecting device including three magnetoresistive elements and two differential operational amplifiers. In this device, two of the three magnetoresistive elements have respective outputs connected to first inputs of the two differential operational amplifiers, and the remaining one of the magnetoresistive elements has an output connected to second inputs of the two differential operational amplifiers in common.
U.S. Patent Application Publication No. 2005/0242802 A1 discloses an angular speed detecting device including three Hall elements which output three output signals different in phase from each other by 90°.
In a rotating field sensor including bridge circuits that use MR elements as magnetic detection elements, ideally, the output signal waveform of each bridge circuit should trace a sinusoidal curve (including a sine waveform and a cosine waveform) as the direction of the rotating magnetic field rotates. As described in U.S. Patent Application Publication No. 2012/0053865 A1, however, the output signal waveform of each bridge circuit is known to be sometimes distorted from a sinusoidal curve. A distortion of the output signal waveform of each bridge circuit may lead to some error in the angle detected by the rotating field sensor. One of the factors that may distort the output signal waveform of each bridge circuit is the MR elements.
The following will describe examples of situations where the output signal waveform of a bridge circuit that uses MR elements is distorted due to the MR elements. Here, assume that the MR elements are giant magnetoresistive (GMR) elements or tunneling magnetoresistive (TMR) elements. GMR and TMR elements each include a magnetization pinned layer whose magnetization direction is pinned, a free layer whose magnetization direction varies depending on the direction of the rotating magnetic field, and a nonmagnetic layer disposed between the magnetization pinned layer and the free layer. One example of the situations where the output signal waveform of the bridge circuit is distorted due to the MR elements is where the magnetization direction of the magnetization pinned layer varies under the influence of the rotating magnetic field or like factors. This is likely to occur when the rotating magnetic field is relatively high in strength. Another example of the situations where the output signal waveform of the bridge circuit is distorted due to the MR elements is where the magnetization direction of the free layer differs from the direction of the rotating magnetic field due to effects such as the shape anisotropy and coercivity of the free layer. This is likely to occur when the rotating magnetic field is relatively low in strength.
Assume here that the output signal of the bridge circuit contains an ideal component which varies periodically in such a manner as to trace an ideal sinusoidal curve, and a signal error which distorts the output signal waveform of the bridge circuit. The signal error is composed mainly of a second harmonic component having a period of ½ the period of the ideal component, and a third harmonic component having a period of ⅓ the period of the ideal component. To reduce an error in the angle detected by the rotating field sensor, it is thus important to reduce the second harmonic component and the third harmonic component.
U.S. Patent Application Publication No. 2012/0053865 A1 discloses a technique for reducing the third harmonic component by providing four detection circuits each of which includes a Wheatstone bridge circuit and performing computations using the output signals of the four detection circuits. This technique, however, requires twice as many Wheatstone bridge circuits as the conventional rotating field sensor which uses two Wheatstone bridge circuits. The aforementioned technique thus has room for improvement in terms of downsizing and structure simplification of the rotating field sensor.
U.S. Patent Application Publication No. 2002/0006017 A1 discloses a technique for correcting a detected angle by establishing electrical connection between a main sensing element having a main reference magnetization axis and two correction sensing elements each having a reference magnetization axis inclined with respect to the main reference magnetization axis. This technique, however, requires that the design of the correction sensing elements be optimized according to the design conditions such as the resistances, sizes and materials of the main sensing element and the correction sensing elements and the strength of the rotating magnetic field, and thus has a drawback that the design of the sensor is not easy.
None of JP S61-173113A, JP H04-005571A and U.S. Patent Application Publication No. 2005/0242802 A1 particularly address reducing the third harmonic component.
As has been described, a rotating field sensor that uses MR elements as magnetic detection elements has a problem that the angle detected by the rotating field sensor may contain some error. This problem can occur in any rotating field sensor that includes magnetic detection elements to detect an angle that the direction of a rotating magnetic field forms with respect to a reference direction.